Chen shi taijiquan developed from Chen family style, tongbei, Shaolin, etc of the day along with synthesis from close family members so inclined to martial endeavours!I would say that it is always ongoing as eveidenced by the laojia/xinjia dajia, etc variations that evolved according to the historical period!

I have no idea ho wlong it would take to finalize but a teacher of mine one toldd me that in his wushu class from a traditional teacher (1950's mainland China), he was to choreograph a form based on a well known taiji style. It took about 6 months but again a template was in place on which to copy!

Zhong,I would have no idea how to even go about creating a Tai Chi form. That's way over my pay grade. It seems to have taken the Yang family six generations of working on their form to get it to where it is today though. So I'd say: Not very quickly. Did you have it in mind to create a form of your own? There is a guy living in the town I live in who has created his own form. He's supposedly a "Master" of Kempo and after taking a few classes of Yang style TCC (he does not say where or with whom) he decided that the form he learned violated Tai Chi principles and so created his own form. He had his Kempo teacher certify him as a "Master of Tai Chi" on the form he created for himself and he now teaches that form at his school.I've seen it. It's a great form...Of Kempo.He's sort of a running joke amongst those who practice traditional Tai Chi around here, actually. I only mention that situation because if you are going to create your own form you're going to want to keep it within the principles of Tai Chi Chuan if you don't want to become the "running joke" amongst the practitioners in your area.

I think that though the exact sequences we see now may be fairly late, the origin of many of the movements goes back to great antiquity, and physical culture traditions including the Daoyin traditions. Here's a reconstructed diagram from a silk ms dating earlier than 200 BC: